As is known, radio frequency (RF) transmitters modulate baseband signals, such as analog voice or digital voice samples, onto an RF carrier, amplify the RF carrier, and transmit the RF carrier, via an antenna, through the air as electromagnetic energy. The electromagnetic energy is subsequently received by a receiver's antenna, demodulated back to the baseband signal, and rendered audible (if voice was transmitted) by the receiver.
A mixer is a three port, time varying network that translates a signal at one frequency to some other frequency. For example, baseband signals may be mixed, i.e., modulated, to an intermediate frequency (IF) and then mixed again to the carrier frequency. In a zero IF transmitter, baseband signals are mixed directly to the carrier frequency. To effect this translation, the baseband signal is heterodyned or mixed in a non-linear device with an RF signal from a local oscillator.
In zero IF transmitters, an upmixer and a selectable attenuator are used to adjust the average power output level of the transmitter. The upmixer draws the same current, no matter what power output is chosen for the selectable attenuator, in order to provide good signal to noise ratio. The upmixer draws the same current whether it is used with a class A amplifier, which draws the same current all the time, a class B amplifier, in which the amplifier current tracks power all the time (in other words, when there is no output power, there is no current draw), or class AB amplifier, where the current draw of the amplifier tracks the power level at high power or peaks, but idles at a fixed current for lower power output.
Accordingly, there is a need for an upmixer that is more efficient at drawing current when less average power is output by the power amplifier than when higher power is output by the amplifier, while providing good signal to noise ratio.